The present invention relates to the field of telecommunications. More particularly, the present invention relates to cellular telecommunications for hearing impaired persons.
For quite some time, Teletype devices (TTY) have been employed in conjunction with conventional telephone systems. In so doing, hearing impaired persons have been able to directly access telephone networks. It should be noted that TTYs that are employed specifically for hearing impaired persons are often referred to as Telecommunications Devices for the Deaf or TDDs. In general, TTYs include a keyboard, or like devices, and a display. Accordingly, a sending party wishing to communicate with a receiving party would then type a string of desired characters (i.e., letters, numbers or other available characters) using the keyboard. Each character is then encoded using a 5 bit coding sequence. A modem then generates a teletype (TTY) signal by converting the bit sequence into a 45.45 bit/sec Frequency Shift Keying (FSK) modulated signal. The TTY signal is then transmitted to the receiving party over the telephone line. A TTY at the receiving end of the telephone line demodulates the TTY signal and decodes the resulting bit sequence into a series of characters, which are displayed for the receiving party.
FIG. 1 illustrates, in more detail, the TTY signal protocol. As stated, each character is encoded into a 5 bit sequence D1-D5. In addition, a stop bit and a start bit are appended to each 5 bit sequence. In order to encode letters, numbers and other available characters using a 5 bit code, two separate character sets are employed. The sending party can then switch back and forth between the two character sets by transmitting a control character, which the sending party accomplishes by depressing one of two dedicated keys on a standard TTY keyboard. The first key is marked LTRS whereas the second is marked FIGS.
With the development of wireless telecommunications, efforts have been undertaken to give hearing impaired persons similar access to wireless networks, such as mobile telecommunications networks, including cellular networks. In fact, in the United States, it is now a requirement that cellular networks have the capability to transport TTY signals. In cellular systems such as the Advanced Mobile Phone System (AMPS), which is an analog system widely employed throughout the United States, TTY signals are transmitted with little or no difficulty. However, transmitting TTY signals over digital cellular systems, such as the Digital AMPS (D-AMPS) and the Global System for Mobile Telecommunications (GSM), is problematic.
The primary reason why transmitting TTY signals over digital cellular systems is problematic is that digital cellular systems employ speech coders. A speech coder, in turn, employs a speech production model which is optimized for analyzing and encoding speech signals based on certain assumptions which are valid only for speech signals, such as the number of spectral peaks in the signal, the maximum rate of change in the spectrum of the signal and the pitch frequency of the signal. However, TTY signals are significantly different from speech signals. Accordingly, speech coders used in conventional, digital cellular systems are not well suited for analyzing and encoding TTY signals. In fact, it has been shown that when TTY signals are analyzed, encoded and transmitted over a conventional digital cellular system, unacceptably high character error rates occur.
Transmitting TTY signals over digital cellular systems is problematic for reasons other than those directly related to the speech coders used in such systems. For instance, TTY signals, like other signals, may be degraded by various channel disturbances such as fading effects and interference. However, in a conventional digital cellular system, the effects of channel disturbances on speech signals can be mitigated by the speech decoder based on the characteristics of the speech signal itself. For example, there is generally a high degree of correlation between the spectrum of consecutive speech frames. Therefore, if the speech decoder detects that a particular speech frame has been lost or severely degraded, the speech decoder might assume that the specific parameters associated with the affected frame, such as the spectral parameters, are identical to the specific parameters associated with the previous speech frame. As the probability of this is relatively high, making this assumption will not, in general, severely impact end-user speech quality. Unfortunately, such a technique would not be suitable for TTY signals, since the information is carried in the spectrum of the signal. With respect to TTY signals, an assumption that one frame will exhibit similar spectral characteristics as those associated with the previous frame will lead to severe degradation of the TTY signal.
In fact, the TTY signal protocol described above includes no error detection or error correction provision. Thus, if one of the 5 bits in a 5 bit TTY signal sequence is erroneous, an incorrect character will, in all likelihood, be displayed at the receiving end. Moreover, the minimum length of time to transmit but one character is 165 msec, as shown in FIG. 1. As each speech coder frame has a length of 20 msec., at least 9 speech coder frames would be required to transmit a single TTY character. A channel error in but one of 9 frames will almost certainly result in a character error. Thus, the character error rate is roughly 9 times the frame erasure rate (FER). For speech signals, a FER of less than 1 percent is generally acceptable. A character error rate 9 times that, however, in a TTY connection is not acceptable.
For the reasons presented above, it would be desirable to provide a method and/or system that permits TTY signals, as well as other, similar low rate data signals, to be effectively transmitted over digital, cellular telecommunications networks. Otherwise, persons having hearing-impairments would be restricted to analog cellular systems, or be forced to accept the relatively poor performance offered by digital cellular systems.
The present invention involves transforming conventional TTY signals so that the resulting signals can be more effectively analyzed and encoded by a speech encoder, and transmitted over a digital cellular system. Although the present invention focuses on TTY signals, the present invention is not restricted thereto. The present invention can be employed to more effectively transmit any low rate data signal over a digital cellular network, such as a dual-tone multiple frequency signal. In general, the present invention accomplishes this by providing a low rate data encoder in the transmission path of the TTY signal, just prior to the speech encoder used by the digital cellular system. In so doing, the FSK modulated TTY signal (i.e., the conventional TTY signal) is transformed into a signal that is more compatible with the speech encoder used in conventional, digital cellular systems. Similarly, a low rate data decoder is provided at the receiving end of the transmission path, after the speech decoder in the digital cellular system has decoded the received signal.
Accordingly, it is an objective of the present invention to improve the coding and transmission performance of low rate data signals over a digital cellular network.
It is also an objective of the present invention to improve the coding and transmission performance of TTY signals in a digital cellular network.
It is yet another objective of the present invention to provide more effective cellular telecommunications services to hearing impaired persons.
It is still another objective of the present invention to improve the coding and transmission performance of low rate data signals, such as TTY signals, in a conventional digital cellular network without having to modify the existing digital cellular network or the digital cellular terminals (e.g., cellular phones).
In accordance with one aspect of the present invention, the above-identified and other objectives are achieved by a system that is capable of encoding and transmitting both speech signals and low rate data signals in a digital cellular network. The system involves a low rate data source that is capable of generating a low rate data signals. A low rate data encoder then receives the low rate data signals and transforms them into modified low rate data signals. A speech encoder is then employed to encode both the speech signals and the modified low rate data signals, prior to signal transmission over a corresponding communications link, wherein the low rate data encoder transforms the low rate data signals into modified low rate data signals so that the low rate data signals are specifically compatible with the speech encoder.
In accordance with another aspect of the present invention, the above-identified and other objectives are achieved by method for employing a speech encoder in a digital cellular telecommunication system to encode and transmit both speech signals and teletype (TTY) signals, where the TTY signals are generated by a telecommunications device for the deaf (TDD). The method involves monitoring an input signal that is to be transmitted over a communication link and determining whether that input signal is a speech signal or a TTY signal. If it is determined that the input signal is a TTY signal, the TTY signal is encoded into a sequence of character codes, where each character code comprises a number of bits and where each character code represents a single character. A channel protection code is then appended to each character code. Thereafter, a modified TTY signal is generated by modulating the sequence of character codes so that the modified TTY signal is compatible with the speech encoder. The modified TTY signal is then forwarded to the speech encoder.